Patent GB-825404-A (GB)
Title: Method for coating titanium dioxide pigment
IPC: C09C1/36
Filed: 1957-04-30
Abstract: Titanium dioxide is subjected to the action of the vapour of a halogen-free non-polymerized organosilicon compound R(4-n)Si(OR1)n where R is alkyl, aryl or hydrogen, R1 is alkyl or hydrogen and n is an integer from 1-3, whereby deposition and polymerization take place to form an organosiloxane coating which renders the material hydrophobic and lyophilic and thus readily dispersible in oil and organic solvents such as acetone and acetonitrile.  Preferably the titanium dioxide is first coated with 0.2-1.7% of a water-insoluble hydrous oxide of a polyvalent metal or metalloid (e.g. aluminium, silicon, titanium or zirconium) and dried before introduction into the vapour; such coatings may be applied by drying a mixed suspension in water of the titanium dioxide and the hydrous oxide.  The siloxane coating may comprise from 0.2-5% of the weight of pigment.  The siloxane coating is preferably prepared during the final milling of the titanium dioxide; the organosilicon monomer may be introduced into the mill as a solid, liquid or vapour and the temperature must be maintained high enough to vaporize the monomer or maintain it in the vapour state.  Such temperatures may vary from room temperature to 250 DEG  C. depending on the compound used, but with higher boiling point compounds having considerable vapour pressures the temperature used may be less than the boiling point.  Organosilicon compounds specified are methyl alkoxysilanes (e.g. trimethylmonoethoxysilane, dimethyldiethoxysilane and monomethyltriethoxysilane), vinyltriethoxysilane, amyltriethoxysilane, ethyltriamoxysilane; arylalkoxysilanes (phenyltriethoxysilane) and silanols (trimethyl and triethyl silanol).ALSO:The vapour of a halogen-free non-polymerized organosilicon compound R(4-n)Si(OR1)n, where R is alkyl, aryl or hydrogen, R1 is alkyl or hydrogen and n is an integer from 1-3, is brought into contact with titanium dioxide whereby deposition and polymerization take place to form an organosiloxane coating which renders the material hydrophobic and lyophilic and thus readily dispersible in oil and organic solvents such as acetone and acetonitrile.  Preferably the titanium dioxide is first coated with 0.2-1.7% of a water-insoluble hydrous oxide of a polyvalent metal or metalloid (e.g. aluminium, silicon, titanium or zirconium) and dried before introduction into the vapour.  The siloxane coating may comprise from 0.2-5% of the weight of pigment.  The siloxane coating is preferably prepared during the final milling of the titanium dioxide; the organosilicon monomer may be introduced into the mill as a solid, liquid or vapour and the temperature must be maintained high enough to vaporize the monomer or maintain it in the vapour state. Such temperatures may vary from room temperature to 250 DEG  C. depending on the compound used, but with higher boiling point compounds having considerable vapour pressures, the temperature used may be less than the boiling point.  Organosilicon compounds specified are: methyl alkoxysilanes (e.g. trimethylmonoethoxysilane, dimethyldiethoxysilane and monomethyltriethoxysilane), vinyltriethoxysilane, amyltriethoxysilane, ethyltriamoxysilane; arylalkoxysilanes (phenyltriethoxysilane) and silanols (trimethyl and triethyl silanol).  It is considered that polymerization takes place due to hydrolysis of the monomer vapour with water adsorbed on the pigment or with a hydrous coating applied thereto.